The best one under $600 is the 70-200/4 IS which is free to you since you already own it. That is unless you have done something less than intelligent.

fuly agree - the 70-200 is the best lens you can get for 200 mm focal length under 600 USD

It's a fantastic lens and it is rigid and lightweight and relatively small - a no-brainer!

I am talking about the IS version though. You see Bob already owns it, the 70-200/4 IS, or at least he did. Bob changes lenses more often than germ freaks wash their hands. So the fact that he is even asking this question implies that Bob is about to do something stupid and give up his best lens, and one of Canon's best lenses, or he already did.